The Student Room Group
Here was my answer to the same question yesterday. Hope it's helpful!

DLB Maths YouTube channel of worked solutions to all past paper questions:
https://www.youtube.com/playlist?lis...pRcajyfstuyARH
Website for worked solutions to the Heinemann textbook and to find past papers:
https://www.highermathematics.co.uk/
For finding questions on specific topics, Dynamic Maths is helpful:
https://www.dynamicmaths.co.uk/Quest...x.php?levelQ=6
Finally, if you like notes on topics:
https://www.hsn.uk.net/higher-maths/notes

I found all of these great and they helped me to get an A1! DLB maths is my favourite. Flashcards with notes and methods really worked for me too. Good luck with it!